One of the factors preventing getting high and stable oats and winter wheat yields in the farms of Belarus is high crops weed infestation. To select herbicides, it is necessary to know the specific composition of weed plants, what gives an opportunity to determine the volumes of treatments and find the most effective herbicides. To study the areas and weed flora specific composition, in 2016-2018 there were itinerary inspections of oats and spring wheat crops in six regions of the Republic (in 3 agroclimatic zones). It is determined that total grass weed infestation round the Republic in 2016-2018 has made, on the average, in wheat crops - 19,2 pcs/m2, in oats crops -14,4. Barnyard grass (Echinochloa crus-galli) has prevailed – 9,5 pcs/m2 in wheat crops and 7,6 – in oats . Also in both crops a perennial grass weed - couch grass (Elytrigia repens) has grown, its average number has been at the level of 4,5 stems/ m2. The share of apera (Apera spica-venti) and wild oats (Avena fatua) has made 10% (1,7 pcs/m2) and 0,5 pcs/m2) , accordingly from total weed number.